A key aspect of retinal physiology is the phagocytosis by the retinal pigmented epithelium of rod outer segment discs shed during the process of rod-renewal. Little is known of the molecular events associated with this process. We shall investigate the hypothesis that carbohydrate components on the membranes of these organelles are involved in recognition phenomena with the cells of the retinal pigmented epithelium as an aspect of their engulfment. Initially, we shall examine a model system of carbohydrate recognition by these cells by investigating their ability to carry out the process of adsorptive endocytosis of glycoproteins which differ in their carbohydrate composition. The biological tool for these studies will be the pigmented epithelium cell of the embryonic chick, maintained in culture; the target molecules will be a series of 125I-neoglycoproteins which differ in the type of carbohydrate present, as well as iodinated, purified lysosomal enzymes with established markers for their uptake by other cells. These studies may reveal the presence in the retinal pigment epithelial cell of recognition systems for the endocytosis of glycoproteins involving specific carbohydrates. The second phase of these studies will investigate the ability of the pigmented epithelium cell, in culture, to engulf bovine rod outer segments, disc membranes, and rhodopsin-liposomes, and the influence on this process of modifying their carbohydrates by chemical and enzymatic means. Special emphasis will be placed on the effect of modifying the carbohydrates of rhodopsin in these membranes. Biochemical, isotopic, and electron-microscopic techniques will be used to follow these reactions.